I had a flowmaster 50 on my 5.4 but it was way too quiet. It sounded nice from the inside and outside when i was really on the gas but not loud enough for me. I got 40 series but after cutting the 50 i realized i got the wrong size so for the passed month ive been driving around with no muffler. At first i thought it sounded sweet but now its just obnoxious. My buddy has air intake and a magnaflow on his 4.6 and i might go in that direction now, it sounds real throaty. Ive heard lots of 40s on 5.7 chevy's though so i might rig something up and give it a try

I was running just the 2 cats on my 4.6 but there was too much interior resonance. So i went and did high flow cats (as mine took a shit) then added single American thunder 40 series in the 2 3/4" size single exhaust setup and sounds alot better with alot less interior drone at highway speeds